There have been widely used optical transmission systems which can transmit a large amount of data at a high speed through a transmission line such as an optical cable in recent years. FIG. 15 is a diagram showing an example of a typical configuration of a conventional optical transmission system. The optical transmission system Zb is divided into a station “A” side and a station “B” side. On the station “A” side are installed a terminal 4A (4) and a transmission apparatus 1Ab (1b), and on the station “B” side are installed a terminal 4B (4) and a transmission apparatus 1Bb (1b). The transmission apparatus 1Ab on the station “A” side and the transmission apparatus 1Bb on the station “B” side are connected with each other through a transmission line 2. The transmission line 2 is a section for an optical communication through an optical cable and the like whose length is from several kilometers to one hundred and several tens kilometers. In addition, each station has a configuration of the terminal 4 and the transmission apparatus 1b to be connected with each other.
The terminals 4 installed on the station “A” side and the station “B” side, generate data to transmit the data to the transmission line 2 through the transmission apparatus 1b, and receive data from the transmission line 2 through the transmission apparatus 1b to process the data. The transmission apparatuses 1b installed on the station “A” side and the station “B” side, receive the data from terminals 4 installed on themselves (i.e., of the own station) to transmit the data to an opposite station through the transmission line 2, and receive the data from the transmission line 2 to transmit the data to the terminal 4 of the own station.
The transmission apparatus 1b includes a transponder unit 10b and a monitoring control unit 20b. The transponder unit 10b receives the data from the terminal 4 to convert the data to optical signals and transmits the optical signals to the transmission line 2, while the transponder unit 10b receives the data from the transmission line 2 to convert the data to digital signals from the optical signals and transmits the digital signals to the terminal 4 of the own station. The transponder unit 10b includes a terminal connection unit 200b, a data processing unit 300b, and an optical-electrical conversion unit 400b. The terminal connection unit 200b is adapted to be in connection with the terminal 4 to transmit and receive the data with the terminal 4. The data processing unit 300b is adapted to receive the data from the terminal connection unit 200b to transmit the data to the optical-electrical conversion unit 400b, and to receive the data from the optical-electrical conversion unit 400b to transmit the data to the terminal connection unit 200b. The optical-electrical conversion unit 400b is connected with the transmission line 2, and is adapted to convert the data transmitted from the terminal 4 to the optical signals and to convert the optical signals received from the transmission line 2 to the digital signals. The monitoring control unit 20b is adapted to monitor the data to be transmitted and received through the transmission line 2.
Here, in one of the transmission apparatuses 1b, an attenuator 5 is installed at the connection portion with the transmission line 2. The attenuator 5 is a so-called attenuator, and is adapted to control levels of the optical signals to be transmitted and received. The optical-electrical conversion unit 400b always outputs the optical signals at a constant level so that the communication can be performed up to a maximum distance requested on the basis of standards of the optical transmission system Zb and specifications of the transmission apparatus 1b. However, the optical-electrical conversion unit 400b may cause deterioration or failure upon receiving an optical signal which has too high optical signal level. When a distance of the transmission line 2 is short enough, the optical signal level is adjusted by the attenuator 5 to be within a receivable optical signal level of the optical-electrical conversion unit 400b on the opposite station side. This means that the transmission apparatuses 1b transmit the optical signals at a level high enough to reach a long distance and attenuate the optical signals by the attenuator 5 when the distance between the transmission apparatuses 1b is short. As shown in FIG. 15, the attenuator 5 can be installed on either end side of the transmission line 2.
In this case, since the transmission apparatuses 1b transmit the optical signals at a level having a considerable margin for the distance of the transmission line 2, unnecessary power can be consumed. In other words, in the optical transmission system Zb, unnecessary power is consumed because the transmission apparatuses 1b attenuate the optical signals by the attenuator 5 after having transmitted the optical signals at a high level.
To solve the above problem, a transmission apparatus and its optical output level control method are disclosed in Patent Document 1. The transmission apparatus on a transmission side transmits a signal at a predetermined optical output level to the transmission apparatus on a reception side, and then an optical output level on the transmission side is determined on the basis of an optical reception level received by the transmission apparatus on the reception side to notify the determined optical output level to the transmission apparatus on the transmission side. This means that, in a technology described in Patent Document 1, the transmission apparatus (parent apparatus) transmits an optical signal once at the beginning. The transmission apparatus (child apparatus) on the opposite side calculates an optical input level margin on the basis of an optical reception level of a received optical signal, and transmits the calculated level margin to the transmission apparatus which is the parent apparatus. The transmission apparatus having received the optical input level margin adjusts the optical output level according to the optical input level margin. In this way, in the technology described in Patent Document 1, the transmission apparatus realizes an energy saving control which refrains from consuming unnecessary power without using an attenuator by reducing the optical output level of the optical-electrical conversion unit in accordance with the distance of the transmission line.